1. Field of the Invention
The present invention relates generally to a mold for molding disc substrata of video discs, compact discs or optical magnetic discs, and more particularly concerns a novel mold for molding the discs in which a diameter of the mold cavity is changed without exchanging the entire mold.
2. Description of Related Art
Disc substrata for compact discs, video discs, or optical magnetic discs of information memory medium (hereinafter called discs) are molded generally in such a way as to inject molten resin into a mold cavity with a stamper which is formed by contacting a movable half-mold fixed to a movable platen of an injection molding machine to a statutory half-mold fixed to a stationary platen. A stamper with numerous pits for information memory is mounted to either the movable half-mold or the stationary half-mold.
There is a standard diameter of the discs. Consequently, when molding discs of different diameters, the diameter of the mold cavity for shaping a disc should be decided in accordance with the standard. The current standard regarding the diameter of discs sets the diameter of the compact disc at 120 mm, of a small compact disc (what is called a compact disc single) at 80 mm., video discs at 200 mm and 300 mm respectively, and optical magnetic discs at 86 mm and 130 mm respectively.
Hitherto, when molding a disc having a diameter different from a diameter applied in a previous injection molding cycle, the whole of the stationary half-mold and the movable half-mold have been dismounted from the stationary platen and the movable platen respectively. It is expensive to provide a new mold for each diameter of the disc.
Also in case of exchanging the stamper (due to difference of information content but not difference of diameter of the disc), it is not necessary to exchange the whole of the mold; however, complicated work of mounting and dismounting the stamper retainer is required for exchanging the stamper, which wastes much labor and time.
In case of molding discs having a diameter different from the diameter applied in a previous injection molding cycle, or in case of molding discs having information different from the information applied in a previous injection molding cycle, both mounting and dismounting the stamper and the outer peripheral stamper retainer have been absolutely required. In the former case exchange of a mirror block with a convex portion is additionally required so as to fit a diameter of the convex portion of the mirror block to a diameter of inner periphery of the stamper retainer.
There is disclosed in the prior art U.S. Pat. No. 4,790,738 referring to exchanging of stamper, plural clamping pins for holding an outer stamper holding means. These are operated by air or hydraulic cylinders. The clamping pins are provided in both of the stationary half-mold and the movable half-mold respectively. These make the mold complicated in operation and in electric circuits and air or hydraulic circuits because of providing the cylinders to both sides of the stationary half-mold and the movable half-mold.